The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Rechargeable batteries are used in many applications. The applications may range from portable electronic devices to industrial equipment. For example, the portable electronic devices may include cell phones, cameras, personal digital assistants (PDAs), laptop computers, and notebook computers. The industrial equipment may include fork-lifts, hybrid-electric vehicles, medical equipment, and uninterruptible power supplies.
Rechargeable batteries typically include cells that utilize different chemical technologies and that generate different output voltages. For example, Nickel-Cadmium (NiCd) and Nickel Metal Hydride (NiMH) cells generate an output voltage of 1.2 volts (1.2V). Lithium ion cells generate output voltages ranging from 3.6V to 3.9V.
Many applications utilize voltages that may be greater than the output voltage generated by a single cell. Accordingly, a battery stack of multiple cells may be used to generate output voltages that are greater than the voltage generated by a single cell. For example, a battery stack comprising two cells may generate an output voltage that can power some portable electronic devices. A battery stack comprising hundreds of cells may generate an output voltage that can power some electric vehicles.
Generally, a cell of a battery has a capacity to store a predetermined amount of charge. The capacity may be called a rated capacity of the cell. An amount of charge remaining in the cell at any time may be expressed in terms of a state of charge of the cell. A cell is in a fully charged state when charged to its maximum capacity (e.g., the rated capacity). Conversely, a cell is in a fully discharged state when discharged to a minimum capacity. The output voltage of the cell is a function of the state of charge of the cell.
Occasionally, a cell may be unable to store charge according to its rated capacity. Instead, the cell may store less charge than its rated capacity. A cell may be called a weak cell or a strong cell based on its ability to store charge according to its rated capacity.
For example, a strong cell can store charge nearly equal to its rated capacity when fully charged. Conversely, a weak cell cannot store charge nearly equal to its rated capacity when fully charged. Instead, the weak cell stores considerably less charge than its rated capacity when fully charged.
When cells are connected in series in a battery stack, the same amount of current flows through the cells during charging and discharging. During charging, a weak cell charges faster than a strong cell and is fully charged before the strong cell. An output voltage of the weak cell reaches its maximum rated value before the strong cell. The weak cell is overcharged when charging is continued to fully charge the strong cell. The output voltage of the weak cell exceeds its maximum rated value when the weak cell is overcharged.
During discharging, the weak cell discharges faster than the strong cell and is fully discharged before the strong cell. The output voltage of the weak cell drops from its maximum rated value faster than the strong cell. The strong cell may reverse charge the weak cell when discharging is continued until the strong cell is fully discharged.
Frequent overcharging and reverse charging adversely impact the number of useful charge-recharge cycles of the cells. Most cells have limited number of useful charge-recharge cycles. For example, lead-acid cells may have 200-500 useful charge-recharge cycles. Nickel-Cadmium (NiCd) cells may have 500-1200 useful charge-recharge cycles. Lithium ion cells may have 300-500 useful charge-recharge cycles. The number of useful charge-recharge cycles is considerably reduced when the cells weaken and are overcharged for a prolonged period of time. Moreover, the cells may be damaged when the weak cells are completely discharged and are reversed charged.
To prevent overcharging and over-discharging of the weak cells, the battery stack may be operated at less than its rated capacity. For example, a charging cycle of the battery stack may be terminated when the weak cell is fully charged. Terminating the charging cycle when the weak cell is fully charged may prevent other cells in the battery stack from fully charging. As a result, the battery stack may supply less power than its rated capacity.
Conversely, a discharge cycle of the battery stack may be terminated when the weak cell is fully discharged. Terminating the discharge cycle when the weak cell is fully discharged may prevent other cells in the battery stack from fully discharging.
Operating the battery stack at less than its rated capacity may result in waste of unused capacity of the battery stack. Additionally, operating the battery stack at less than its rated capacity may increase the number of charge-recharge cycles.
Instead, each cell of the battery stack may be monitored individually. The charging and discharging of each cell may be controlled to prevent damage to the weak cells. For example, controllable dissipative bypass devices may be used with each cell. A controller that controls charging and discharging may sense when a weak cell is fully charged. The controller may turn on a dissipative bypass device associated with the weak cell when the weak cell is fully charged. The dissipative bypass device bypasses the weak cell from further charging while charging of other cells continues until their rated capacities are reached. Thus, the dissipative bypass device prevents overcharging of the weak cell.
Additionally, the controller may sense when the weak cell is nearly fully discharged. The controller may disable further discharging of the battery stack when the weak cell is nearly fully discharged. Thus, the controller may prevent over-discharging of weak cell.
This approach protects the weak cells from being overcharged and over-discharged. However, the useful capacity of the strong cells is not available for utilization. Further, using dissipative bypass devices reduces round-trip charge/discharge efficiency during charging.